Apple touts the slow motion video capability in its new iPhone 5s, equipped with an upgraded iSight camera and a powerful 64-bit A7 processor. It turns out, as a little experiment has shown, that it might not be a true 720p slow-mo with 120fps, but rather upscaled from 480p.
The experiment was done against the new Galaxy Note 3 from Samsung, which features true 720p slow motion effect in its video camera settings. The regular 720p video captured with the Note 3 shows noticeably more detail than the allegedly 720p one from the iPhone 5s in the same circumstances, as you can see from the video screenshots below.
When the Note 3 video was upconverted to 720p, too, the resulting loss of detail was pretty similar to what we get with Apple's default slow-mo settings, which led to the thought that this is precisely what Apple is doing here. In any case, the best clues as to why Apple might be doing this are to save on resources - like memory, for instance.